Adaptive baking system and method of using the same

ABSTRACT

An adaptive baking system includes a baking chamber configured to receive a wafer, and a heating element configured to support the wafer. The adaptive baking system further includes a controller configured to receive temperature information related to the heating element and the wafer, wherein the controller is further configured to adjust an amount of heat provided by the heating element during a baking process in response to the temperature information.

BACKGROUND

Wafer baking is used to help cure a photoresist material on a waferprior to patterning the photoresist material. The wafer is placed in abaking chamber and heated using a heating element. A duration andtemperature of the baking process are predetermined based on a materialof the photoresist material and characteristics of the wafer. Thetemperature in the baking chamber is kept at a constant temperatureduring the baking process. Following the baking process, the photoresistis patterned.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a schematic diagram of an adaptive baking system in accordancewith some embodiments.

FIG. 2 is a top view of a heating element in accordance with someembodiments.

FIG. 3 is a graph of a temperature of a wafer and a heating elementduring a baking process in accordance with some embodiments.

FIG. 4 is a flow chart of a method of using an adaptive baking system inaccordance with some embodiments.

FIG. 5 is a block diagram of a general purpose computer system forimplementing instructions for using an adaptive baking system inaccordance with one or more embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

FIG. 1 is a schematic diagram of an adaptive baking system 100 inaccordance with some embodiments. Adaptive baking system 100 includes achamber 110. A heating element 112 is positioned within chamber 110 andconfigured to support a wafer 114. Wafer 114 has a curvature whichresults from warpage induced by prior processing steps performed on thewafer. A gap 116 exists between wafer 114 and heating element 112 due tothe warpage. During operation, heating element 112 is configured togenerate heat in order to bake wafer 114. A controller 120 is configuredto provide signals to heating element 112 to control an amount of heatprovided to wafer 114. Adaptive baking system 100 further includessensors 118 embedded in a surface of heating element 112.

Chamber 110 is configured to house wafer 114 on heating element 112.Chamber 110 includes a port for inserting and removing wafer 114 from aninterior of the chamber. In some embodiments, chamber 110 includes aninsulating covering configured to reduce heat loss from walls of thechamber to a surrounding environment. In some embodiments, walls ofchamber 110 include steel, aluminum, ceramic or other suitable material.In some embodiments, chamber 110 is sufficiently large to house multipleheating elements 112.

Heating element 112 is configured to support wafer 114 and supply heatto the wafer during the baking process. Heating element 112 isconfigured to receive signals from controller 120 and change an amountor a location of heat provided to wafer 114 during the baking process.In some embodiments, heating element 112 includes sensors configured tomeasure a temperature at a surface of the heating element closest towafer 114. In some embodiments, the sensors include thermometers,thermocouples, or other suitable temperature measuring elements. In someembodiments, heating element 112 includes a resistive heater. In someembodiments, heating element 112 includes at least one channel fortransporting a heating fluid through the heating element. In someembodiments, heating element 112 includes an infrared light sourceconfigured to emit infrared radiation to contact wafer 114.

FIG. 2 is a top view of a heating element 200 in accordance with someembodiments. Heating element 200 includes thirteen heating zones 1-13.In some embodiments, the heating element includes from 2 to 100 zones orfrom 5 to 30 zones. In some embodiments, the heating element as from 1to 5 heating zones per 50-80 cm² of the heating element surface. In someembodiments, heating element 200 includes more or less than thirteenheating zones. In some embodiments, the heating zones are arranged in aconcentric pattern. In some embodiments, the heating zones are arrangedin a radial pattern. In some embodiments, the heating zones are arrangedin a linear pattern or any other suitable arrangement. In someembodiments, the heating zones are individually controllable to adjustan amount and a duration of heating provided to a wafer, e.g., wafer 114(FIG. 1).

In some embodiments where heating element 200 includes resistive heatingelements, at least one separate heating element is included in eachheating zone. In some embodiments where heating element 200 includes achannel for transporting a heating fluid, each heating zone includes aflow controlling element configure to enable increased or decreased flowof the heating fluid through the channel in the respective heating zone.In some embodiments, the flow controlling element includes a diverter, avalve, or another suitable flow controlling element. In some embodimentswhere heating element 200 includes an infrared light source, eachheating zone includes radiation blocking element configured toselectively enable propagation of the infrared radiation through theheating element. In some embodiments, the radiation blocking elementincludes a shutter, a liquid crystal element, an electro-chromic elementor another suitable radiation blocking element.

Returning to FIG. 1, during operation wafer 114 is housed in chamber 110and supported on heating element 112. In some embodiments, wafer 114includes active elements. In some embodiments, wafer 114 includes aphotoresist layer over a surface thereof. In some embodiments, thebaking process is used to cure the photoresist layer over wafer 114.

In some embodiments, chamber 110 includes sensors configured to measurea temperature of at least one surface of the wafer 114. In someembodiments, the sensors are configured to connect to wafer 114 prior toinsertion into chamber 110. In some embodiments, the sensors areconfigured to connect to wafer 114 following insertion into chamber 110.In some embodiments, the sensors, e.g., sensors 118, are embedded in asurface of heating element 112. In some embodiments, the sensors areremote sensors configured to measure a temperature of wafer 114 withoutcontacting the wafer. In some embodiments, the sensors includethermometers, thermocouples, or other suitable temperature measuringelements.

Gap 116 is between wafer 114 and heating element 112. In someembodiments, where wafer 114 experiences little or no warpage duringprior processing gap 116 is reduced or omitted. Gap 116 changes anability of heating element 112 to transfer heat to wafer 114 by changinga medium through which the heat energy is transferred. Edges of wafer114 are in direct contact with heating element 112 while gap 116 has agreatest thickness near a center portion of the wafer. Based on thisarrangement, the edges of wafer 114 will receive a higher amount of heatenergy in comparison with the center portion of the wafer if heatingelement 112 is configured to provide uniform heating across an entiresurface of the heating element.

Controller 120 is configured to control an amount of heat energyprovided by heating element 112. In some embodiments, controller 120 isconfigured to receive temperature information from wafer temperaturesensors. In some embodiments, controller 120 is configured to receivetemperature information from heating element temperature sensors. Insome embodiments, controller 120 is configured to individually controlheating zones of heating element 112.

In some embodiments where controller 120 is configured to individuallycontrol heating zones, the controller is configured to send a singleheating signal to heating element 112. Heating element 112 thenaddresses the signal to the heating zones based on address informationin the signal. In some embodiments where controller 120 is configured toindividually control heating zones, the controller is configured to senda separate signal to each heating zone of heating element 112. In someembodiments, controller 120 is configured to control individual heatingzones in based on an amount of heat provided in adjacent heating zones.For example, control for heating zone 3 of heating element 200 (FIG. 2)is controlled based on an amount of heat provided in adjacent heatingzones 1, 2, 4, 7 and 8.

In some embodiments, controller 120 is configured to control heatingelement 112 based on a graph of a temperature of heating element 112 andof a temperature of wafer 114. FIG. 3 is a graph 300 of a temperature ofa wafer and a heating element during a baking process in accordance withsome embodiments. Graph 300 includes a plot 310 of a target heatingelement temperature and a plot 320 of a measured heating elementtemperature. Graph 300 further includes a plot 330 of a target wafertemperature and a plot 340 of a measured wafer temperature. A largestheating element temperature variation 350 exists between plot 310 andplot 320. A largest wafer temperature variation 360 exists between plot330 and plot 340. A compensation heating duration 370 is used to modifya baking process to obtain satisfactory results for a wafer, e.g., wafer114 (FIG. 1).

In some embodiments, a controller, e.g., controller 120 (FIG. 1) isconfigured to control compensation heating duration 370. In someembodiments, as an area between plot 310 and plot 320 or an area betweenplot 330 and plot 340 increases, the controller is configured toincrease compensation heating duration 370. In some embodiments, aslargest heating element temperature variation 350 or largest wafertemperature variation 360 increases, the controller is configured toincrease compensation heating duration 370. In some embodiments, as amagnitude of a slope of plot 310 increases, the controller is configuredto decrease compensation heating duration 370. The controller isconfigured to adjust a size of compensation heating duration 370 bysending signals to a heating element, e.g., heating element 112. In someembodiments, the controller is configured to adjust an amount of heatprovided by the heating element uniformly across the heating element. Insome embodiments, the controller is configured to adjust an amount ofheat provided by the heating element based on individual heating zoneson the heating element.

FIG. 4 is a flow chart of a method 400 of using an adaptive bakingsystem in accordance with some embodiments. Method 400 begins withoperation 402 in which a wafer is inserted into a baking chamber. Thewafer is positioned on a heating element, e.g., heating element 112(FIG. 1), in the baking chamber. In some embodiments, the wafer, e.g.,wafer 114, is inserted through a port in an outer wall of the bakingchamber, e.g., chamber 110. In some embodiments, the wafer is insertedusing a robotic arm. In some embodiments, the wafer is stored in a frontopening universal pod (FOUP) prior to insertion in to the bakingchamber. In some embodiments, the port is configured to close followinginsertion of the wafer. In some embodiments, multiple wafers arepositioned on a single heating element. In some embodiments, the bakingchamber includes multiple heating elements and a single wafer is placedon each heating element.

Method 400 continues with operation 404 in which a baking processbegins. In some embodiments, the baking process is used to cure aphotoresist layer on the wafer. In some embodiments, the baking processbegins by using a controller, e.g., controller 120 (FIG. 1), to providea signal to the heating element. Heating element begins providing heatto the wafer in response to the signal from the controller. In someembodiments, an additional heating element is used to provide a portionof the heat provided to the wafer. In some embodiments, the additionalheating element comprises a radiative heating element, a heating gasinserted into the baking chamber, a heating lamp or another suitableheating element. In some embodiments, a temperature of the bakingprocess is determined based on a curing temperature of a layer on thewafer. In some embodiments, the temperature of the baking process isbased on empirical data collected from previous baking processes.

In operation 406, a temperature of the wafer and a temperature of theheating element are monitored. In some embodiments, the temperature ofthe wafer is monitored using a plurality of sensors positioned across asurface of the wafer. In some embodiments, the temperature of the waferis monitored using a remote temperature sensing element. In someembodiments, the sensors are connected to the wafer prior to operation402. In some embodiments, the sensors are connected to the waferfollowing operation 402. In some embodiments, the sensors are embeddedin a surface of the heating element. In some embodiments, the sensorsinclude thermometers, thermocouples, or other suitable temperaturemeasuring elements.

In some embodiments, the temperature of the heating element is monitoredusing a plurality of sensors positioned across a surface of the heatingelement. In some embodiments, the temperature of the heating element ismonitored using a remote temperature sensing element. In someembodiments, the sensors are embedded in a surface of the heatingelement. In some embodiments, the sensors include thermometers,thermocouples, or other suitable temperature measuring elements. In someembodiments, the sensors on the wafer are located to correspond to aposition of the sensors on the heating element. In some embodiments, thesensors on the wafer are located at positions which do not correspond toa position of the sensors on the heating element.

The temperature information related to the wafer and the heating elementis provided to a controller. In some embodiments, the controller isconfigured to directly receive the temperature information. In someembodiments, the controller is configured to receive the temperatureinformation from a relay or decoding element electrically between thesensors of at least one of the wafer or the heating element and thecontroller.

Method 400 continues with operation 408 in which a temperature of theheating element is controlled. The controller receives the temperatureinformation, determines whether a temperature of the wafer or theheating element is different from a target temperature and generatesinstructions for adjusting the temperature of the heating element basedon this determination. In some embodiments, the controller extends atime of the baking process based on the determined temperaturevariation.

In some embodiments, the controller individually controls heating zonesof the heating element. In some embodiments, the controller sends asingle heating signal to the heating element, the heating signal is thenaddressed to a specific heating zone by the heating element. In someembodiments the controller sends a separate signal to each heating zoneof the heating element. In some embodiments, the controller controlsindividual heating zones in based on an amount of heat provided inadjacent heating zones. For example, control for heating zone 1 ofheating element 200 (FIG. 2) is controlled based on an amount of heatprovided in adjacent heating zones 2-5.

In operation 410, the baking process ends and the wafer is removed fromthe baking chamber. In some embodiments, the wafer is removed through aport in an outer wall of the baking chamber. In some embodiments, thewafer is removed through a same port through which the wafer is insertedin operation 402. In some embodiments, the wafer is removed through adifferent port from the port through which the wafer is inserted inoperation 402. In some embodiments, the wafer is removed using a roboticarm. In some embodiments, the wafer is stored in a FOUP followingremoval from the baking chamber. In some embodiments, the port isconfigured to close following removal of the wafer.

FIG. 5 is a block diagram of a general purpose computer system 500 forimplementing instructions for using an adaptive baking system inaccordance with one or more embodiments System 500 includes a hardwareprocessor 502 and a non-transitory, computer readable storage medium 504encoded with, i.e., storing, the computer program code 506, i.e., a setof executable instructions. Computer readable storage medium 504 is alsoencoded with instructions 507 for interfacing with manufacturingmachines for producing the memory array. The processor 502 iselectrically coupled to the computer readable storage medium 504 via abus 508. The processor 502 is also electrically coupled to an I/Ointerface 510 by bus 508. A network interface 512 is also electricallyconnected to the processor 502 via bus 508. Network interface 512 isconnected to a network 514, so that processor 502 and computer readablestorage medium 504 are capable of connecting to external elements vianetwork 514. The processor 502 is configured to execute the computerprogram code 506 encoded in the computer readable storage medium 504 inorder to cause system 500 to be usable for performing a portion or allof the operations as described in method 400.

In some embodiments, the processor 502 is a central processing unit(CPU), a multi-processor, a distributed processing system, anapplication specific integrated circuit (ASIC), and/or a suitableprocessing unit.

In some embodiments, the computer readable storage medium 504 is anelectronic, magnetic, optical, electromagnetic, infrared, and/or asemiconductor system (or apparatus or device). For example, the computerreadable storage medium 504 includes a semiconductor or solid-statememory, a magnetic tape, a removable computer diskette, a random accessmemory (RAM), a read-only memory (ROM), a rigid magnetic disk, and/or anoptical disk. In some embodiments using optical disks, the computerreadable storage medium 504 includes a compact disk-read only memory(CD-ROM), a compact disk-read/write (CD-R/W), and/or a digital videodisc (DVD).

In some embodiments, the storage medium 504 stores the computer programcode 506 configured to cause system 500 to perform method 400. In someembodiments, the storage medium 504 also stores information needed forperforming a method 400 as well as information generated duringperforming the method 400, such as a target wafer temperature parameter520, a wafer temperature parameter 522, a target heating elementtemperature parameter 524, a heating element temperature parameter, aheating element layout parameter and/or a set of executable instructionsto perform the operation of method 400.

In some embodiments, the storage medium 504 stores instructions 507 forinterfacing with manufacturing machines. The instructions 507 enableprocessor 502 to generate manufacturing instructions readable by themanufacturing machines to effectively implement method 400 during amanufacturing process.

System 500 includes I/O interface 510. I/O interface 510 is coupled toexternal circuitry. In some embodiments, I/O interface 510 includes akeyboard, keypad, mouse, trackball, trackpad, and/or cursor directionkeys for communicating information and commands to processor 502.

System 500 also includes network interface 512 coupled to the processor502. Network interface 512 allows system 500 to communicate with network514, to which one or more other computer systems are connected. Networkinterface 512 includes wireless network interfaces such as BLUETOOTH,WIFI, WIMAX, GPRS, or WCDMA; or wired network interface such asETHERNET, USB, or IEEE-1394. In some embodiments, method 400 isimplemented in two or more systems 500, and information such as wafertemperature, heating element temperature and heating element layout areexchanged between different systems 500 via network 514.

System 500 is configured to receive information related to a targetwafer temperature through I/O interface 510 or network interface 512.The information is transferred to processor 502 via bus 508, theinformation is then stored in computer readable medium 504 as targetwafer temperature parameter 520. System 500 is configured to receiveinformation related to wafer temperature through I/O interface 510 ornetwork interface 512. The information is stored in computer readablemedium 504 as wafer temperature parameter 522. System 500 is configuredto receive information related to the target heating element temperaturethrough I/O interface 510 or network interface. The information isstored in computer readable medium 504 as target heating elementtemperature 524. System 500 is configured to receive information relatedto heating element temperature through I/O interface 510 or throughnetwork interface 512. The information is stored in computer readablemedium 504 as heating element temperature parameter 526. System 500 isconfigured to receive information related to heating element layoutthrough I/O interface 510 or network interface 512. The information isstored in computer readable medium 504 as heating element layoutparameter 528.

During operation, processor 502 executes a set of instructions todetermine a difference between the target wafer temperature and thewafer temperature as well as between the target heating elementtemperature and the heating element temperature. Processor 502 isconfigured to use this difference to determine whether to adjust anamount heat being provided by the heating element. Processor 502 isconfigured to use the heating element layout parameter 528 to determinewhich heating zone(s) to increase or decrease an amount of heatprovided. In some embodiments, processor 502 is configured to extend anamount of time of a baking process based on the determined temperaturevariation. In some embodiments, processor 502 is configured to increasean amount of heat provided by the heating element as a temperaturevariation between the target wafer temperature and the wafer temperatureor between the target heating element temperature and the heatingelement temperature increases. In some embodiments, processor 502 isconfigured to increase an amount of heat provided by the heating elementas a largest heating element temperature variation between the targetwafer temperature and the wafer temperature or between the targetheating element and the heating element temperature increases. In someembodiments, processor 502 is configured to decrease an amount of heatprovided by the heating element as a rate of change of the temperatureof the heating element versus time increases.

One aspect of this description relates to an adaptive baking system. Theadaptive baking system includes a baking chamber configured to receive awafer, and a heating element configured to support the wafer. Theadaptive baking system further includes a controller configured toreceive temperature information related to the heating element and thewafer, wherein the controller is further configured to adjust an amountof heat provided by the heating element during a baking process inresponse to the temperature information.

Another aspect of this description relates to a method of using anadaptive baking system. The method includes supporting a wafer on aheating element, wherein the heating element is located in a bakingchamber. The method further includes heating the wafer for a firstduration using the heating element, and measuring a temperature of theheating element and a temperature of the wafer during the firstduration. The method further includes adjusting an amount of heatprovided by the heating element during the first duration based on thetemperature of the heating element or the temperature of the wafer.

Still another aspect of this description relates to a controller for anadaptive baking system. The controller includes a non-transitorycomputer readable medium configured to store information related to atarget temperature of a wafer, a target temperature of a heatingelement, a temperature of the wafer, and a temperature of the heatingelement. The controller further includes a processor connected to thenon-transitory computer readable medium, the processor configured togenerate at least one heating signal during a baking process to adjustan amount of heat provided by the heating element to the wafer.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. An adaptive baking system comprising: a bakingchamber configured to receive a wafer; a heating element configured tosupport the wafer; and a controller configured to receive temperatureinformation, wherein the temperature information includes a temperatureof the heating element and a temperature of the wafer, and wherein thecontroller is further configured to adjust an amount of heat provided bythe heating element during a baking process in response to thetemperature information, and wherein the controller is configured todecrease the amount of heat provided by the heating element as a rate ofchange of the temperature information versus time increases.
 2. Theadaptive baking system of claim 1, wherein the heating element comprisesa plurality of heating zones.
 3. The adaptive baking system of claim 2,wherein the controller is configured to individually control eachheating zone of the plurality of heating zones.
 4. The adaptive bakingsystem of claim 2, wherein the controller is configured to control afirst zone of the plurality of heating zones based on an amount of heatprovided by heating zones adjacent to the first heating zone.
 5. Theadaptive baking system of claim 1, wherein the controller is configuredto increase a duration of the baking process in response to thetemperature information.
 6. The adaptive baking system of claim 1,wherein the heating element comprises at least one temperature sensorembedded in a surface of the heating element configured to support thewafer.
 7. The adaptive baking system of claim 1, wherein the controlleris configured to provide a single heating signal to the heating elementfor adjusting the amount of heat provided by the heating element.
 8. Theadaptive baking system of claim 1, wherein the controller is configuredto provide a plurality of heating signals to the heating element foradjusting the amount of heat provided by the heating element.
 9. Theadaptive baking system of claim 8, wherein the heating element comprisesa plurality of heating zones and each heating zone of the plurality ofheating zones is configured to receive a corresponding heating signal ofthe plurality of heating signals.
 10. The adaptive baking system ofclaim 1, wherein the heating element comprises: at least one channel forreceiving a heating fluid; and at least one flow controlling elementconfigured to control a flow of the heating fluid through the at leastone channel, wherein the controller is configured to control the atleast one flow controlling element.
 11. The adaptive baking system ofclaim 1, wherein the heating element comprises: at least one infraredlight source configured to illuminate the wafer; and a plurality ofshutters for controlling propagation of radiation from the at least oneinfrared light source, wherein the controller is configured to controleach shutter of the plurality of shutters.
 12. The adaptive bakingsystem of claim 1, wherein the heating element comprises: a plurality ofresistive heating elements, wherein the controller is configured tocontrol each resistive heating element of the plurality of heatingelements.
 13. An adaptive baking system comprising: a baking chamberconfigured to receive a wafer; a heating element configured to supportthe wafer; and a controller configured to receive temperatureinformation including a temperature of the heating element and atemperature of the wafer, wherein the controller is further configuredto adjust an amount of heat provided by the heating element during abaking process in response to the temperature information, and toincrease a duration of an entirety of the baking process in response tothe temperature information; and wherein the controller is configured todecrease the amount of heat provided by the heating element as a rate ofchange increases, and the rate of change is a time-based change of thetemperature information.
 14. An adaptive baking system comprising: abaking chamber configured to receive a wafer; a heating elementconfigured to support the wafer, wherein the heating element comprises aplurality of heating zones; and a controller configured to receivetemperature information including a temperature of the heating elementand a temperature of the wafer, wherein the controller is furtherconfigured to: adjust an amount of heat provided by the heating elementduring a baking process in response to the temperature information,decrease the amount of heat provided by the heating element in responseto an increase in a rate of change of the temperature information,control a first zone of the plurality of heating zones based on anamount of heat provided by heating zones adjacent to the first heatingzone, and increase a duration of an entirety of the baking process inresponse to the temperature information.
 15. The adaptive baking systemof claim 14, wherein the controller is configured to individuallycontrol each heating zone of the plurality of heating zones.
 16. Theadaptive baking system of claim 14, wherein the controller is configuredto provide a single heating signal to the heating element for adjustingthe amount of heat provided by the heating element, and the heatingelement is configured to route the single heating signal to a specificheating zone of a plurality of heating zones of the heating elementbased on information in the single heating signal.
 17. The adaptivebaking system of claim 1, wherein the controller is configured toincrease a duration of an entirety of the baking process based on alargest temperature difference between a target wafer temperature and ameasured wafer temperature.
 18. The adaptive baking system of claim 1,wherein the controller is configured to increase a duration of anentirety of the baking process based on a largest temperature differencebetween a target heating element temperature and a measured heatingelement temperature.
 19. The adaptive baking system of claim 13, whereinthe controller is configured to increase the duration of the bakingprocess for an entirety of the wafer based on a largest temperaturedifference between a target wafer temperature and a measured wafertemperature.